MFC r261150, r261151, r261164, r261166, r261167, r261168, r261394, r261395:

 - Read and write the MAC address in the config space byte by byte
 - Also include the mbuf's csum_flags in an assert message
 - Remove stray space
 - Move duplicated transmit start code into a single function
 - Avoid queue unlock followed by relock when the enable interrupt race
   is lost
 - Check for a full virtqueue in the multiqueue transmit path
 - Do not place the sglist used for Rx/Tx on the stack
 - Use m_defrag() instead of m_collapse() to compact a long mbuf chain